1. Field of the invention
The present invention relates to a vertebral fixing system adapted to be mounted on a vertebra, and to a spine-straightening assembly using the system.
One field of application that is envisaged, is particularly but not exclusively, the treatment of scoliosis or, more generally, the correction of abnormal curvatures of the spine.
2. Description of the Related Art
The spine is formed of superposed vertebrae, normally aligned along a vertebral axis, from the lumbar vertebrae to the cervical vertebrae, and each having a posterior wall from which projects a spinous process and two lateral edges from the walls of which there project ribs and/or transverse processes. If the spine of a person has abnormal curvature, the vertebrae are inclined relative to each other and relative to said vertebral axis. The lateral edges of the vertebrae on one side are therefore closer together and form a concave shape while the lateral edges on the other side are farther apart and form a convex shape.
To straighten the vertebral column, the lateral edges of the vertebrae on the concave side are moved away from one another and supported at distances from one another substantially equivalent to the distances between the lateral edges on the other side. Devices known in the art are used thereafter to hold the vertebrae relative to one another, and comprise screws that are inserted into the vertebrae or hooks that are inserted along the internal wall of the spinal canal and rods adapted to connect the screws or hooks.
Pairs of hooks are generally inserted into each vertebra, one on each side, near the pedicle; they have heads that project from the posterior wall of the vertebra, one on each side of the spinous process. The heads are tulip-shaped, for example, and are adapted to receive a rod that is immobilized by a nut that is screwed onto the head and bears on the rod. The heads of the hooks situated on either side of the spinous process are connected together and fixed in position by two rods parallel to each other and to the axis of the spine.
However, using such hooks is difficult because the operative must not under any circumstances touch the spinal cord that extends along the centre of the spinal canal, since that would result in paralysis of the patient.
Using screws reduces the risks of the procedure. The screws also have tulip-shaped heads and are inserted in pairs into the pedicles on each side of the spinous process on the posterior wall of the vertebrae. The screws therefore constitute fixing points on the vertebrae for holding them relative to one another. However, they must be inserted into the pedicles of the vertebrae, which in some cases are small or have deteriorated.
A problem that arises, and that the present invention aims to solve, is that of providing fixing points when it is not possible to insert screws into the vertebrae in the abnormal curvature region and when using hooks is too dangerous.